Use username instead of email for signups

It is very common for a user (#1) to signup/pay for one or more other users (#2, #3 etc.) - all of whom already have site accounts.

The problem is that while #1 may know the USERNAME(s) for the additional attendee(s) they're paying for (or can find via site directory/member list, for instance), they typically won't know and cannot find, or be sure of their correct account EMAIL. Therefore...

Out of frustration or for expediency, they enter bogus or INcorrect email(s) for the additional attendee(s) they're paying for - thereby creating duplicate account(s) for those attendees.

I was thinking that function: uc_signup_attendees_form_emails could be altered to ask for and accept USERNAME(s) instead of EMAIL(s).